Vacuum assisted wound dressing

ABSTRACT

Apparatus for the application of topical negative pressure therapy to a wound site is described, the apparatus comprising: a wound contacting element for retaining wound exudate fluid therein; a wound covering element that provides a substantially airtight seal over the wound contacting element and wound site; a vacuum connection tube connecting a wound cavity to a vacuum source; and a vacuum source connected to a distal end of the vacuum connection tube.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser. No. 12/744,055, filed May 20, 2010, which is a U.S. National Phase of the PCT International Application No. PCT/GB2008/051088 filed on Nov. 20, 2008, designating the United States and published on May 28, 2009 as WO 2009/066104, which claims priority to Great Britain Patent Application No. 0722820.8, filed Nov. 21, 2007. The disclosure of these prior applications is incorporated by reference in their entirety and should be considered a part of this specification.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a topical negative pressure (TNP) assisted wound dressing particularly, though not exclusively, for the treatment of wounds of relatively low area and/or relatively low volume.

The subject of this invention is an apparatus for the management of small to medium sized wounds that utilises a vacuum source but manages wound exudate in a traditional manner by utilising an absorbent self-cohesive material in the wound cavity. No fluid is exported from the locality of the wound cavity other than by local evaporation. In this manner, an extremely portable system, of minimal inconvenience to the wearer, can be generated.

Background to the Invention

TNP therapy has enjoyed relatively recent commercial success. WO9605873 and its family members describes a portable TNP therapy apparatus. The apparatus described mechanically supports tissue in the vicinity of the wound site and tissue mechanics and the rate of exudation from such sites requires a system such as that described in WO9605873 or as described in patents such as GB2378392 and WO2005/082435 which have remote waste receptacles to which wound exudate fluid is aspirated in order to cope with the volume of fluids generated in a relatively short time, less than that period in which a dressing would normally be left in place. However, for wounds of surface area below approximately 200 cm² or internal volumes below about 1000 cm³ these solutions may not be the most appropriate since exudate volumes and exudate rates from these wounds may be managed by more traditional wound dressings, requiring dressing change every 3-7 days. The relatively small dimensions of such wounds do not make them attractive for the traditional TNP therapies disclosed in the prior art; these devices typically including a remote vacuum source, fluid transfer lumen and remote fluid collection receptacle, control and power source and are of dimensions and weight exceeding those convenient or discrete for the patient to carry.

The general principles of TNP apparatus described in the prior art comprise a fluid-permeable wound cavity filling element, a dressing for covering the wound and providing a reasonably air-tight seal around the wound, a drainage tube connecting the wound site and cavity filling element to the vacuum source via a fluid collection canister. The precise nature of the wound filling element has been the subject of much invention in this field. The mode of action of the apparatus is the application of negative pressure to the wound cavity, causing compression of the wound cavity filler and expansion of the surrounding tissue into the wound cavity. Wound exudate is drawn from the surrounding tissue, through the still porous cavity filler, along the drainage tube and into the remote collection receptacle. An important feature of the prior art is the ability of the wound cavity filler to remain sufficiently porous, when compressed under negative pressure, to allow fluid transport from the tissue to the drainage or aspirant tube. Porosity can be facilitated at the molecular level, for example in a hydrogel, or at the microscopic level, for example in a hydrocellular foam. To facilitate fluid flow, a hydrophobic filling has been deemed particularly desirable by workers in the field and absorbent fillers as being particularly undesirable due to their hindering fluid transport.

In contrast to the principles of TNP therapy, the general principle of traditional wound dressings is the localisation of wound exudate at the locality of the wound, either within the wound cavity or in close proximity to the surface. For this purpose, extremely absorbent materials are desirable that retard the free flow of fluid, preferably absorbing the fluid and localising it. Aquacel (trade mark) made by ConvaTec Ltd is an example of a non-woven dressing that absorbs substantial quantities of fluid and effectively locks it in the dressing. Allevyn (trade mark) made by Smith & Nephew Ltd is an example of a foam dressing that absorbs substantial quantities of fluid while allowing rapid transpiration through a high moisture vapour permeable top-film.

SUMMARY OF SOME EXEMPLIFYING EMBODIMENTS

In summary, the prior art deals exclusively with vacuum assisted fluid transport away from the site of the wound. A very broad range of wound cavity filling and contacting elements have been described and exemplified in the prior art, including materials commonly used in traditional wound care dressings. Without exception in these cases, the cavity filling and wound contacting elements act as a conduit for the transport of fluid from the wound per se to a remote collection canister via an aspirant tube connected to a vacuum source.

An object of the present invention is to overcome or reduce the limitations of the prior art for the management of wounds of low surface area, particularly those below approximately 200 cm² or internal volumes below 1000 cm³, while not resorting to the exclusive use of traditional absorbent dressings. A further object of the present invention is to overcome or minimise the problem of vacuum device portability.

According to a first aspect of the present invention there is provided apparatus for the application of topical negative pressure therapy to a wound site, the apparatus comprising: a wound contacting element for retaining wound exudate fluid therein; a wound covering element that provides a substantially airtight seal over the wound contacting element and wound site; a vacuum connection tube connecting the wound contacting element to a vacuum source; and a vacuum source connected to a distal end of the vacuum connection tube.

The wound contacting element essentially blocks liquid transport beyond itself under pressures between atmospheric pressure and 200 mmHg below atmospheric pressure. Preferably, the wound contacting element material blocks liquid transport beyond itself at pressures of up to 250 mmHg below atmospheric pressure.

This invention concerns apparatus including a dressing for the management of wounds. In contrast to current therapies and prior art in the field of TNP therapy, the invention provides a system that, while exposing the wound to the many benefits of TNP, does not allow the export of wound fluid from the confines of the wound cavity. The apparatus relies upon a wound contacting element that does not allow the transport of fluid beyond itself under the range of negative pressures being applied to the wound.

A particular advantage of the apparatus according to the present invention is that confinement of wound fluid to the immediate vicinity of the wound enables the provision of an extremely small and light and consequently highly portable vacuum source and a convenient simple coupling and decoupling means of the vacuum source to the wound site dressing and which overcome significant limitations in the usability and portability of prior art apparatus.

In the present specification the term ‘wound contacting element’ means the portion of the apparatus/dressing filling the wound cavity or covering the wound. The nature of the wound contacting element is not restricted, provided that its composition or structure is able to essentially block the flow of exudate away from itself under the pressure range specified above.

For example, the wound contacting element may include a liquid-triggered valve that closes when it becomes in contact with a liquid. Such a valve may be situated proximate to the vacuum tube connection point at the wound covering element. Thus, when contacted by liquid the valve closes and no liquid is transported by the vacuum tube. The valve may be an electromechanical valve or a smart valve comprised of a water-absorbent material that expands to close within a constriction upon contact with liquid. The water-absorbent material may be placed within a restricted aperture within the valve. The wound contacting element positioned between such a valve and the wound may be absorbent or non-absorbent but may preferably be absorbent. The filling, in this embodiment, may be any medically-suitable composition such as, for example, a gauze, a foam, a woven, a non-woven, knit or moulded material.

Alternatively, the wound contacting element may comprise entirely, or in part, a suitable material structured such that it can absorb wound exudate but does not allow transmission of this fluid to the vacuum tube. This configuration is defined by two parameters: the aperture of the exit(s) from the material constituting the wound contacting element and the mechanical integrity of the material when wet. For example, an absorbent material that becomes laden with exudate must not itself be displaced along the vacuum connection tube. This is a particular problem with particulate or fibrous so-called superabsorbent materials, for they can pass, even when fully saturated with fluid, through very small apertures of a size below which vacuum levels cannot be efficiently maintained. In effect this means that the narrower the tube transmitting the vacuum, the greater the loss in vacuum pressure with distance from the vacuum source, which loss in negative pressure is negligible for macroscopic bores but can be significant as bores reduce below 1 mm diameter. To overcome these problems, an absorbent material with substantially enhanced self-cohesive properties compared to those currently available is described in our co-pending patent application GB0719065.5 of common ownership herewith.

When an absorbent material is included in the wound contacting element, the absorbent material not necessarily acting as a fluid transport blocking element, the material may preferably be capable of absorbing more than 5-times its own weight in fluid, more preferably more than 10-times its own weight in fluid and more preferably still more that 15-times its own weight in fluid. Such high w/w fluid absorbency may be desirable so that the wound can be initially dressed with a low weight material thereby reducing stress on the wound and the patient.

One group of materials particularly suited for this purpose are so-called superabsorbent materials, for example, those based on polycationic or polyanionic polymers. Superabsorbent polyanionic polymers include polyacrylic acid salts and polyacid derivatives of polysaccharides, such as carboxyalkylcellulose, or structural derivatives. Preferably, when the material is polyanionic, it may be a polyacrylic acid salt or derivative or carboxymethylcellulose or derivative. Preferably, when the material is polycationic, it may be chitosan-based, more preferably a carboxyalkylchitosan or derivative, even more preferably carboxymethylchitosan.

One particularly preferred material is a superabsorbent material capable of self-coalescence upon fluid absorption (see our GB0719065.7 the content of which is included herein by reference). These materials are able to effectively block the transport of liquid beyond their boundaries and also do not themselves flow or disaggregate under the influence of negative pressure or at the levels of externally applied physical stresses resulting from the negative pressure within a wound cavity.

A preferred material attribute may be the ability to achieve rapid haemostasis in the event of bleeding in the wound site.

A further preferred material attribute may be the ability to kill pathogens, such as bacteria or fungi, which come into contact with it. Preferably the material is inherently antimicrobial.

Carboxyalkylchitosan-based materials are suitably both haemostatic and antimicrobial.

The wound contacting element material can be provided in any form suitable to enable fluid ingress and absorption but to block the flow of fluid away from the wound contacting element. Suitable designs include dispersions of superabsorbent particles within a network of wicking fibres (as utilised in diapers, for example) or reticulated or discontinuous material comprising the superabsorbent material, such as open celled foams, knits, laminates, woven or non-woven materials. Preferably, the material may be in the form of a non-woven sheet for application to largely two-dimensional wounds or non-woven balls for application to largely three-dimensional wound cavities.

The wound covering element may be any material substantially impermeable to the flow of liquid, but may or may not be substantially permeable to the transmission of water vapour. The wound covering element is preferably a highly conformable transparent material which may optionally be coated with, or be manufactured such that, the side contacting the wound contact element and the patient's skin may be considered adhesive to the skin. Here, adhesive is taken to mean able to stay in place in the absence of negative pressure. Suitable materials for the manufacture of a highly conformable transparent wound covering include polyurethanes, polyolefins, polyacrylates, silicone-based polymers or composites comprising any combination of these materials.

The wound covering element may be a traditional wound dressing, for example composed of an absorbent foam, for example, Allevyn (trade mark) made by Smith & Nephew Medical Limited or non-absorbent film such as Tegaderm (trade mark) made by 3M Inc.

The wound covering element may be optionally provided with a means of connecting the vacuum connection tube with the wound covering element by, for example, a central or radial aperture or apertures in the wound covering element. The wound contacting element may be connected, via the wound covering element, to the vacuum connection tube by any means known to the skilled person including luer fittings such as commercially available valves and ports, magnetic couplings or adhesive sheet or tape. The connection of the vacuum tube to the wound covering element may preferably be achieved via a non-adhesive elastomeric cup positioned at the end of the vacuum connection tube and a pressure-sensitive valve positioned in the wound covering element itself. The pressure-sensitive valve may open when a pressure differential of a specified magnitude exists over its two surfaces such as between 5 and 200 mmHg, for example. The material comprising the elastomeric cup and pressure-sensitive valve may preferably be a silicone-based material. It is desirable that the coupling system is suitable for repeated coupling and uncoupling, as convenient for the patient.

Optionally, these coupling elements may be impregnated or coated with antimicrobial materials including, but not restricted to, antibiotics, silver compounds or materials, iodine-based formulations, polyhexamethylene biguanide, triclosan or chlorhexidine. Preferably, the elements may be coated with silver clusters.

The vacuum connection tube may be any of appropriate mechanical properties and bore for the transmission of negative pressure from the vacuum source to the wound contacting element. However, dependent upon the configuration of the wound contacting element, the bore of the tube may be such that it is incapable of transmitting dry or hydrated components of the wound contacting element. The tubing may be as conformable and light weight as possible and may be coiled or linear. The tubing may be single or multi-lumen, or a combination of lumens, and may optionally split and or rejoin to form separate tubular elements for the management of a single wound site or multiple wound sites. The tubing may be opaque or transparent.

The wound contacting element and wound covering element may be optionally combined into a single element.

The wound covering element and vacuum connection tube may optionally be combined into a single element.

The wound contacting element, wound covering element and vacuum connection tube may be optionally combined into a single element.

The vacuum source may be any available and may be optionally mechanically powered by, for example, a compressed spring and comprise a syringe as the vacuum generating means as is known in the prior art; or electrically powered, for example a vacuum pump. Preferably, the vacuum source may be capable of generating vacuums in the range of −10 mmHg to −250 mmHg relative to atmospheric pressure. More preferably, the vacuum source is a vacuum pump capable of generating vacuums in the range of −10 mmHg to −250 mmHg relative to atmospheric pressure. The pump may generate a vacuum by any convenient means; diaphragm pumps, peristaltic pumps, Venturi-effect pumps and other displacement pumps may be suitable for this purpose.

The vacuum source may preferably be below 500 g in weight, more preferably below 100 g in weight and even more preferably below 50 g in weight.

In cases where the vacuum source is electrically powered, the power source may be mains supplied, a battery supply or a locally generated supply such as a clockwork generator, a solar cell, a thermo cell or a kinetic autorelay type of power source. Preferably the power source may be a battery.

In cases where the power source is a battery, the battery may be disposable or rechargeable. When the battery is rechargeable, recharging may be achieved via a charging station for the vacuum housing or for the battery itself. Battery life may be preferably longer than 12 hours, more preferably longer than 24 hours and even more preferably longer than 72 hours. The battery may preferably be below 100 g in weight, more preferably below 50 g in weight.

The power and vacuum sources may be housed separately or together. Preferably they are housed together. When housed, the combined weight of the power and vacuum source and housing may be preferably less than 1 kg, more preferably less than 500 g, more preferably still less than 200 g. The housing may be of any geometry but is preferably adapted so as to be convenient for the patient and/or carer to handle and carry. It may also preferably be of dimensions below 15×15×6 cm.

The apparatus of the first aspect of the present invention may be applied to wounds for their management. The general principle of the apparatus is the application of the wound contacting element to the wound, covering the wound contacting element with the wound covering element and coupling the wound contacting element to a vacuum source via a vacuum connection tube. As mentioned above, two or more of these elements may be provided as a single entity. Preferably for largely two-dimensional wounds, the wound contacting element and the wound covering element may be a single entity. This combined entity may contain the pressure-sensitive valve in its top surface and may be attached to the perimeter of the wound by appropriate means. Attachment of the dressing to a patient may be achieved by the application of negative pressure alone, in the absence of a bonding means or may be achieved via a bonding means. Attachment may preferably be achieved by a bonding means, for example a pressure sensitive adhesive applied to the skin contacting surface of the wound covering element. When the bonding means is a pressure sensitive adhesive, it may preferably be a poly(acrylate)- or silicone-based formulation.

According to a second aspect of the present invention there is provided apparatus for the application of topical negative pressure therapy to a wound site, the apparatus comprising: a wound covering element that provides a substantially airtight seal over the wound; a vacuum connection tube connecting the wound covering element to a vacuum source; a vacuum source connected to a distal end of the vacuum connection tube; the wound covering element having valve means associated therewith which permits only fluid flow out of a wound cavity defined by the wound covering element and the wound.

When the wound contacting element and the wound covering element are in place, the vacuum source is activated and the vacuum connection tube, preferably attached to the vacuum source, may be connected to the wound covering element via an aperture or valve in the wound covering element. At any point during the application of negative pressure therapy, the coupling of vacuum connection tube to the wound covering element can be reversibly broken and re-established at the convenience of the patient or carer.

In an embodiment of the apparatus according to either the first or second aspects of the present invention the dressing wound covering element may comprise a one-way valve as mentioned above, the valve essentially being able to allow fluid in the form of air to be withdrawn from the wound cavity defined by the wound covering element via the vacuum connection tube. As mentioned above the vacuum connection tube may preferably be repeatably connectable and disconnectable to the dressing/wound covering element without damage thereto such that the vacuum source may be removed by the patient and the dressing left in place but sealed against the ingress of bacteria and potential infection, for example, by the presence of the one-way valve in the wound covering element. The one-way valve means may be a simple plastics material self-sealing valve as are available commercially for many diverse applications outside of the field of TNP therapy such as those sold under the trade mark “miniValve” by Mini Valve International, for example.

Desirably the vacuum connection tube may be attached to the dressing/wound covering element by non-adhesive means so as to facilitate repeated connection/disconnection thereof without damaging the wound covering element film material. In this regard the vacuum connection tube may be connected by “sucker” means at the dressing end of the vacuum connection tube, the sucker means being, for example, in the form of a cup-shaped, domed or bell-shaped conformable plastics material member which is in fluid communication with the vacuum connection tube and which member which may be placed over the valve means in the wound covering element and seal with surrounding wound covering element material, the sucker means being held in place on the dressing/wound covering element by the vacuum generated by the vacuum source itself. Disconnection of the vacuum connection tube may then be effected merely by turning off the power source to the vacuum source or by breaking the seal of the sucker by lifting its edge (NB the sucker can easily be removed by this intentional manipulation but cannot easily be accidentally dislodged by vertical extensive force e.g. pulling on the vacuum connection tube).

When the wound contacting element becomes saturated with wound exudate, the vacuum connection tube can be disconnected and the wound contacting element and wound covering element (or a physical combination of the two) can be exchanged for a new set.

Wounds suitable for management by the apparatus that is the subject of this invention include injuries to soft and hard tissue, including the skin, muscle, cartilage, tendons, bone and internal organs.

In the second aspect of the invention a wound contacting element of highly absorbent material may not be present and wound exudate may be aspirated from the wound cavity to a remote waste receptacle by the vacuum source.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be more fully understood examples will now be described by way of illustration only with reference to the accompanying drawings, of which:

FIG. 1 shows a schematic cross section of an embodiment of apparatus according to the present invention and a wound being treated by TNP therapy;

FIGS. 2A to 2C which show the arrangement of FIG. 1 simplified and the progression of saturation of a wound contacting element; and

FIG. 3 illustrates a liquid contact triggered valve.

DETAILED DESCRIPTION OF SOME EXEMPLIFYING EMBODIMENTS

Referring now to the drawings and where the same features are denoted by common reference numerals.

FIG. 1 shows a schematic cross section through a wound 10 having an embodiment of apparatus 12 according to the present invention applied to it for the purpose of TNP therapy of the wound. The wound 10 has a wound contacting element 14 placed in the cavity defined by the wound, the wound contacting element being roughly level with surrounding sound skin 16. A wound covering element 18 is applied over the wound to contact and seal with the surrounding sound skin by means of a layer of pressure sensitive adhesive (not shown) coated onto the skin contacting surface of the wound covering element material. The wound covering element 18 has an aperture 20 in the area above a part of the wound and the wound contacting element 14. A one-way valve 22 is positioned in the aperture 20 to permit fluid in the form of air to be extracted from the wound cavity 24 defined by the wound covering element 18 and the wound 10 itself. A vacuum source 26 in the form, in this example, of a battery powered vacuum pump is connected to the wound cavity by a vacuum connection tube 28 and a cup-shaped connection member 30 having an aperture 32 therein to accept the end of the vacuum connection tube 28. The cup-shaped member 30 has a flange portion 34 which seats onto the upper surface 36 of the wound covering element 18 material and seals therewith. The valve 22 has an orifice 40 which is normally closed due to the resilience of the material from which it is moulded, for example, silicone plastics based material. The orifice 40 is in the form of a slit normally closed by two lips 42, 44 (the shape of the valve orifice may be likened to a flat bladed screwdriver and the valve orifice 40 in FIG. 1 runs normal to the plane of the page). The valve 22 is initially held in and sealed to the wound covering element material 18 by a flange portion 46 bearing against the underside of the wound covering element 18 material and a circular shoulder 48 and recess 50 which is held in the aperture 20. To evacuate the wound cavity 24, the vacuum pump 26 is started and the cup-shaped member 30 placed on the wound covering element material above the valve 22 and the reduction in pressure within the wound cavity 24 causes the cup-shaped member 30 to be sealed securely to the dressing by the excess pressure of the surrounding ambient air. As the vacuum in the wound cavity develops, the wound covering element 18 is pushed down against the wound contacting element 14 by ambient air pressure and the wound contacting element 14 compressed against the wound surface to apply TNP therapy thereto.

Whilst it is perfectly feasible for the lower surface of the flange portion 34 of the cup-shaped member 30 to be adhesively coated and to be so retained on the wound covering element material, in this example retention of the vacuum connection tube 28 to the wound dressing is solely by ambient air pressure as described above. In case the patient wishes to detach the vacuum pump 26 and leave it behind this may simply be achieved by turning off the pump 26 and allowing the vacuum to degrade and removing the cup-shaped connection member 30. In this case the valve 22 is self-sealing prevents access of bacteria and the like to the wound cavity 24.

The valve 22 exemplified above is a miniValve (trade mark) supplied by Mini Valve International. However, this valve is merely exemplary and many other types of suitable valves are available commercially.

For example International Patent Application No PCT/EP2008/063046 discloses a composition which coalesces on hydration and which can be used to provide a valve which closes upon contact with liquid. The application is included fully herein by way of reference but briefly discloses a suitable composition that enables a new physical transformation. The physical transformation in question involves the conversion of a first stable physical geometry into a second stable physical geometry upon hydration, wherein hydration enables the self-coalescence (fusion) of spatially separated elements or surfaces of the first stable physical geometry.

Each geometry is physically stable. Thus, immersion of the first stable physical geometry in excess solution results in conversion to the second stable physical geometry without significant loss of the material mass by dissolution. That is, the second stable geometry is insoluble, or has only very limited solubility, in the excess solution. The second stable physical geometry is, at least substantially, self supporting such that it is able to retain its shape when is excess solution, or when removed therefrom. In typical preferred forms in the second stable physical geometry the material of the invention is a gel or gel like material.

A feature of the composition is the physical homogeneity of the object in both the first and second physical geometries.

The novel transformation is enabled by construction of the object, at least in part, from materials that can exist in physically stable forms in the dry state and the hydrated state. Furthermore, the hydrated state of the material must be sufficiently self-cohesive, even when immersed in excess solvent, to enable fusion to occur. This, we believe, is a property unique to a limited range of states of matter, some of which we prepare to exemplify this invention.

In broad terms the composition of matter when formed into an object of suitable geometry, can self-coalesce upon hydration in a suitable solvent.

According to a first aspect of the composition there is provided a high molecular mass cationic polymer material having a first state which includes at least two separate but adjacent surfaces and a second state in which the polymer consists of a homogeneous body, wherein the material transitions from the first state to the second state upon hydration.

Thus, on hydration the material expands and the surfaces merge or coalesce to result in a body of self supporting material, typically a gel or gel-like material, which has uniform properties in any dimension. Surfaces and other boundaries within the body of material are absent. Furthermore the body of material is insoluble, or at least of limited solubility in the hydrating solvent and is able to retain its physical geometry under leading (for example gravity).

The term ‘suitable geometry’ is taken to describe an arrangement where separate (for example spatially separate, but not necessarily physically separate) elements or surfaces of the object are sufficiently proximate to enable coalescence upon hydration-induced expansion.

The term ‘suitable solvent’ is taken to describe a fluid (liquid or gas) that can be absorbed be the object, causing expansion and a change in the physical properties of the object (e.g. surface energy). The suitable solvent is typically and preferably an aqueous medium.

The term ‘self-coalesce’ is taken to describe the transformation of two or more spatially separated physically homogeneous elements into a single physically homogeneous element or of fusion of previously spatially separated surfaces of the same element.

Suitable compositions of matter from which objects can be formed are those comprised, entirely or in part, of high average molecular weight cationic polymers including zwitterionic (carrying both anionic and cationic charge) polymers with a cationic charge bias. The cationic polymer may be, or may be a derivative of, a synthetic or a naturally occurring polymer. Preferably, the cationic polymer is one carrying amine functionality. More preferably, the cationic polymer is a polysaccharide. More preferably still, the cationic polymer is chitosan or a derivative of chitosan. The chitosan may be derived from any source, marine or fungal, and is preferably of a weight average molecular weight (Mw) exceeding 10 kDa (kilodaltons), more preferably exceeding 100 kDa and most preferably exceeding 200 kDa.

Where the polymer is a derivative of chitosan, it is preferably a carboxylated derivative. More preferably, it is a carboxyalkyl derivative of chitosan. More preferably still, it is a carboxymethyl derivative of chitosan. The carboxymethyl derivative of chitosan is preferably of a weight average molecular weight exceeding 50 kDa, more preferably exceeding 100 kDa, especially exceeding 500 kDa, more especially exceeding 600 kDa and especially 700 kDa or more.

Carboxymethylation is preferably achieved using known reagents: a base and chloroacetic acid or preferably a neutral salt of chloroacetic acid such as sodium chloroacetate. Preferably, the reaction is carried out in a single step: chitosan fibres or (less preferably) particles being immersed in a solution of reagents or vice versa. Suitable reaction solvents include mixtures of an alcohol with water. The alcohol may be any known but is preferably a non-solvent for chitosan and carboxymethylchitosan, for example isopropanol. The base may be any known but is preferably a water-soluble inorganic base such as sodium hydroxide or potassium hydroxide, preferably sodium hydroxide.

According to a second aspect of the composition there is provided a method of preparing high molecular mass carboxymethylchitosan comprising the steps:

-   -   a. mixing chitosan with a solution of a base and chloroacetic         acid, or a neutral salt thereof, dissolved in a reaction solvent         comprising a mixture of an alcohol and water;     -   b. allowing the reaction to proceed at ambient temperature for         at least 8 hours whilst ensuring adequate exposure of the         chitosan to the reaction solvent;     -   c. when the reaction is complete, washing the reaction product         in excess alcohol-containing solvent;         wherein the volume (in millilitres) of the reaction solvent is         at least 20-times the mass (in grams) of chitosan.

A high molecular mass carboxymethyl chitosan preferably comprises a carboxymethyl chitosan having a mass of at least 500 kDa, more especially at least 600 kDa and especially 700 kDa or more.

In one preferred embodiment the volume of reaction solvent (in millilitres) exceeds the mass of chitosan (in grams) by more than 20 but less than 70-times, more preferably by more than 30-times but less than 40-times.

In another preferred embodiment the mass of sodium chloroacetate exceeds the mass of chitosan by not more than 2-times, more preferably by not more that 1.2-times.

In a preferred embodiment, the alcohol of the reaction solvent is isopropanol.

In further preferred embodiments the reaction is carried out at ambient temperature for a period of at least 8 hours, more preferably for at least 15 hours and even more preferably for at least 18 hours.

In a particularly preferred embodiment, the alcohol of the reaction solvent is isopropanol, the mass of sodium chloroacetate is not more than twice (more especially not more than 1.2 times) the mass of the chitosan and the reaction is allowed to proceed for at least 8 hours.

When the chitosan is provided for reaction in powder or fibre form, this material should be adequately exposed to the turbid reaction solvent throughout the duration of the reaction. This process can be facilitated by any means known to the artisan but can be simply achieved by rolling the reaction vessel, for example.

When the reaction is complete, reaction by-products detrimental to the stability of the product, such as sodium chloride or sodium hydroxyacetate, should be removed to the maximum extent feasible. To achieve this, the reaction product is washed, preferably in one or more steps, in excess solvent comprised of at least 60 parts alcohol (such as ethanol) and 40 parts water (60:40).

More than one washing step is preferred and, when this is the case, the first wash step has preferably a higher water content than subsequent steps, with water content decreasing in every wash step. For example, a suitable two-step wash procedure involves a first wash in excess solvent comprised of at least 60 parts ethanol and 40 parts water (60:40) and a second wash in excess solvent comprised of at least 90 parts ethanol and 10 parts water (90:10).

Thus in a preferred embodiment the reaction product is washed in a plurality of washing stages, each employing an excess of a solvent comprising alcohol and water, wherein in each succeeding stage the solvent consists of a higher proportion of alcohol. Preferably the alcohol is ethanol

It is essential that wash solvents always includes some water to avoid excessive dehydration of the product, which can result in brittleness.

The composition of the wash solvent may include any suitable alcohols such as ethanol, isopropanol or methanol. Ethanol is preferred.

The product resulting from washing and solvent removal can be sterilised by methods typical for the sterilisation of medical devices, for example gamma-irradiation, electron-beam irradiation or ethylene oxide treatment.

Prior to radiation-based sterilisation, the washed reaction product should be adequately solvent-free. This can be achieved by any drying process known to the skilled artisan. A preferred drying process is conducted at temperatures not exceeding 40° C., more preferably not exceeding 30° C. Preferably, solvent removal is achieved by placing the material under a sub-atmospheric pressure. The pressure is preferably less than 500 mbar, more preferably less than 1000 mbar. The duration of the drying process, when achieved by vacuum drying, preferably exceeds 8 hours, more preferably exceeding 12 hours.

The weight average molecular weight of the material following washing and radiation sterilisation is preferably greater than 120 kDa, more preferably greater than 130 kDa and after washing and ethylene oxide sterilisation is preferably greater than 400 kDa, more preferably greater than 500 kDa. It is important that these molecular weights are obtained to avoid mechanical integrity problems in the final product and dissolution problems when exposed to fluid.

Additives and co-components can be added at any stage of the above process, prior to terminal sterilisation. These agents may be any suitable for a topical or internal medical application, such as analgesics, anaesthetics, antimicrobial agents, anti-cancer agents, nicotine or nicotine substitutes or other synthetic or naturally-derived pharmaceuticals including peptides, proteins such as growth factors or retardants, enzymes (e.g. those facilitating tissue debridement), DNA or RNA fragments.

When the additive is an antimicrobial agent, it may be for example: silver or silver compounds, iodine or iodine compounds, quaternary amine-based antimicrobials such as polyhexamethylenebiguanide or chlorhexidene, antibiotics such as gentamycin, vancomycin or a peptide-based agent.

When silver is introduced into the formulation, and the formulation is carboxymethylchitosan-based, addition is preferably achieved by immersion in a solvent mixture of a similar composition as that applied during the carboxymethylation process.

In a third aspect, the composition can be used to provide a method of fusing two or more solid surfaces, wherein the surfaces are initially separate (in particular, spatially separated) but adjacent surfaces of one or more object(s) comprising a self-coalescing material as herein described, notably the high molecular mass polymer material of the first aspect of the invention. The method comprises the step of immersing said surfaces in an aqueous medium and thus hydrating and expanding the self-coalescing material. In one embodiment, the surfaces are initially spatially separated surfaces of the same object. Alternatively, the surfaces are initially spatially separated surfaces of different objects. These alternatives are not mutually exclusive. The surfaces may be the surfaces of fibres, for example in a woven or, more especially, a non-woven fibrous material. In such materials, the surfaces may have portions which are spaced apart and portions which, while being separate, are in contact.

Objects fabricated from the compositions defined above, and suitable for the method, need to be suitably designed to enable coalescence upon hydration. For example, an isolated linear object would not have the opportunity to self-coalesce upon hydration. In contrast, a pair of isolated but adjacent linear objects would have the opportunity to swell and coalesce upon hydration. In this context, ‘adjacent’ means located within about 10 mm of one another. Thus, suitable objects can be defined as containing, at least in part, spatially separated elements or surfaces located within about 10 mm of one another. Preferably, the spatially separated elements or surfaces are located within 5 mm of one another. More preferably, the spatially separated elements or surfaces are located within 1 mm of one another. In some cases, for example fibre based materials, at least parts of adjacent surfaces may be in contact.

Preferred physical formats that meet the above description are fibre-based materials such as woven and non-woven materials. Other suitable formats include knits, open-celled foams and laminates including corrugated materials. More complex arrangements can be fabricated by methods known to one skilled in the art, such as lithography, micromachining and electrospinning. The composition and its uses is not restricted to formats of high open area but includes solid monoliths. Fibre based materials are preferred and fibre-based non-woven materials are particularly preferred.

The composition in use is not restricted to objects consisting exclusively of self-coalescent material, but includes composites, for example composites of common medical device formats and self-coalescent material and surface-coatings, for example implantable metal- or biomaterial based devices including soft-tissue substitutes and joint implants. Composites suitable for topical and internal wound management include those combining polyurethane based materials, such as foams, slabs and films with self-coalescent materials, for example in powdered or, more especially, fibrous form.

When devices comprised, at least in part, of the compositions are immersed in a fluid, they absorb fluid, become swollen and self-coalesce across contact points. Use is not restricted to specific compositions or specific fluids, but in preferred forms and for preferred end-sues, the fluid is most preferably water based. For example, in the case of carboxymethylchitosan-based materials, the fluid is preferably water based. Examples of water based fluids include water or a solution of water, such as saline or a biologically-derived fluid such as whole blood, blood plasma, serum, saliva, wound exudate or bone marrow aspirate.

The novel material properties of the described self-coalescing materials can be exploited in a range of applications, for example in irreversible fluid valving systems and moulding materials.

EXAMPLES Example 1

Generation of Self-Coalescing Carboxymethylchitosan Fibres

A) Synthesis

Immediately prior to reaction, sodium chloroacetate (1.75 g) was dissolved in 4% aqueous sodium hydroxide solution (7 ml). This solution was added to isopropanol (45 ml) and shaken vigorously, resulting in a turbid suspension. This mixture was added to a vessel containing chitosan fibres (1.50 g), the container sealed and rolled at approximately 60 rpm for 18 hours.

B) Washing Steps

B1) After step A, the fibres were removed from the now clear reaction solvent and transferred to a vessel containing 99:1 ethanol:water (200 ml). The material was disturbed every 15 minutes for 1 hour, after which time the material was removed and physically dried by the application of hand pressure between several layers of absorbent material. Following gross drying, the material was vacuum dried at ambient temperature overnight.

B2) After step A, the fibres were removed from the now clear reaction solvent and transferred to a vessel containing 60:40 ethanol:water (200 ml). The material was disturbed every 15 minutes for 1 hour, after which time the material was removed and transferred to a second vessel containing 90:10 ethanol:water (200 ml). The material was disturbed every 15 minutes for 1 hour, after which time the material was removed and physically dried by the application of hand pressure between several layers of absorbent material. Following gross drying, the material was vacuum dried at ambient temperature overnight.

Example 2

Generation of Self-Coalescing Carboxymethylchitosan Fibres (Scale-Up)

Immediately prior to reaction, sodium chloroacetate (96.8 g) was dissolved in 4% aqueous sodium hydroxide solution (387 ml). This solution was added to isopropanol (2490 ml) and shaken vigorously, resulting in a turbid suspension. This mixture was added to a vessel containing chitosan fibres (83.0 g), the container sealed and rolled at approximately 60 rpm for 18 hours. After this time, the fibres were removed from the now clear reaction solvent and transferred to a vessel containing 99:1 ethanol:water (2000 ml). The material was disturbed every 15 minutes for 1 hour, after which time the material was removed and physically dried by the application of hand pressure between several layers of absorbent material. Following gross drying, the material was vacuum dried at ambient temperature overnight.

Example 3

Radiation Sterilisation of Self-Coalescing Carboxymethylchitosan Fibres

The material resulting from Example 1, step B2 was packaged in gas-permeable sterilisation pouches and sterilised by gamma irradiation at 30-40 kGy. The molecular weight of the material pre- and post-sterilisation was determined by gel permeation chromatography. The molecular weight prior to sterilisation was approximately Mw 700 kDa; the molecular weight post-sterilisation was approximately Mw 140 kDa. The molecular weight change in the material, although substantial, was such that the physical properties of the material were not significantly altered by sterilisation.

Example 4

Ethylene Oxide Sterilisation of Self-Coalescing Carboxymethylchitosan Fibres

The material resulting from Example 1, step B2 was packaged in gas-permeable sterilisation pouches and sterilised by ethylene oxide treatment. The molecular weight of the material pre- and post-sterilisation was determined by gel permeation chromatography. The molecular weight prior to sterilisation was approximately Mw 700 kDa; the molecular weight post-sterilisation was approximately Mw 575 kDa. The molecular weight change in the material was such that the physical properties of the material were not significantly altered by sterilisation.

Example 5

Water Absorbency of Self-Coalescing Carboxymethylchitosan Fibres

The material resulting from Example 3 (100 mg) was immersed in water (4 ml) for 1 minute and withdrawn. Excess liquid was allowed to drain and then the hydrated transparent mass was weighed. The material was calculated to absorb approximately 25-times its own mass in water without significant dissolution.

Example 6

Serum Absorbency of Self-Coalescing Carboxymethylchitosan Fibres

The material resulting from Example 3 (100 mg) was immersed in serum (4 ml) for 1 minute and withdrawn. Excess liquid was allowed to drain and then the hydrated transparent mass was weighed. The material was calculated to absorb approximately 13-times its own mass in serum without significant dissolution.

Example 7

Self-Coalescence of Carboxymethylchitosan Fibres in Water

The material resulting from Example 3 (100 mg) was immersed in water (4 ml) for 1 minute and withdrawn. Excess liquid was allowed to drain and then the hydrated transparent mass was allowed to stand for 4 hours. After this time, the individual fibres of the material had self-coalesced and the material was then effectively a homogeneous, elastic hydrogel, able to stably retain its physical geometry under loading (FIG. 2).

Example 8

Self-Coalescence of Carboxymethylchitosan Fibres in Serum

The material resulting from Example 3 (100 mg) was immersed in serum (4 ml) for 1 minute and withdrawn. Excess liquid was allowed to drain and then the hydrated transparent mass was allowed to stand for 4 hours. After this time, the individual fibres of the material have self-coalesced and the material as effectively a homogeneous, elastic hydrogel, able to stably retain its physical geometry under loading.

FIGS. 2A to 2C show stages in the absorption of wound exudate into the wound contacting element 14. The basic arrangement may be the same as that of FIG. 1, however, the drawings of FIG. 2 have been simplified. FIG. 2A shows a clean dressing newly installed in the wound 10 and without any exudate being taken up; FIG. 2B shows the wound contacting element 14 partially full of exudate; and, FIG. 2C shows the wound contacting element 14 saturated with wound exudate. However, the nature of the wound contacting element material 14 is such that no liquid is aspirated by the vacuum connection tube 28 out of the wound cavity 24 or indeed out of the wound contacting element 14 itself.

FIG. 3 shows paradigms of the conversion of a first stable physical geometry into a second stable physical geometry upon hydration, wherein hydration enables the self-coalescence (fusion) of spatially separated elements of the first stable physical geometry. In case A, the self-coalescence comprises the fusion of spatially separated surfaces of a single element; in case B the surfaces are adjacent surfaces of two separate elements.

The Examples described below are based on the use of the apparatus arrangement shown in FIG. 1 and/or FIG. 2 and/or FIG. 3.

Example 1

Application of apparatus of FIG. 1 to ex vivo cavity wound. A 5 cm diameter, 5 cm depth cavity wound was cut by scalpel into a porcine leg joint. The musculature in the area of the wound cavity was injected with saline to ensure that the tissue was adequately hydrated for the duration of the experiment. The wound cavity was packed with two wound contacting elements of non-woven balls of carboxymethylchitosan fibre and the wound cavity and filling was covered over with an adhesive silicone gel sheet CicaCare (trade mark) made by Smith & Nephew Medical Ltd with a central 5 mm diameter aperture 20. A luer loc fitting attached to a coiled vacuum hose was inserted through the aperture and connected to a battery-powered vacuum source Pac-Vac (trade mark) made by Virtual Industries Inc. Immediate contraction of the wound margin was observed and the non-woven balls were compressed down to be flush with the surface of the skin. The system was left in place for 8 hours undisturbed, after which time no fluid had exited the cavity packing but had collected within it.

Example 2

Assembly of apparatus based on FIG. 1 having a wound covering element 14 of Allevyn Adhesive (trade mark) made by Smith & Nephew Medical Ltd.

A 3 mm diameter aperture in the top film of Allevyn Adhesive was created using a biopsy punch. Over this aperture was bonded a silicone elastomer dome miniValve (trade mark) made by Mini Valve International B. V., part number DO 072.004. A miniature vacuum source made by Virtual Industries Inc., PAC-VAC V3200 (trade mark) was coupled by vacuum tubing 28 to a hand-made silicone rubber cup 30.

Example 3

Usage of apparatus based on Allevyn Adhesive as in Example 2. A 5 cm diameter, 5 cm depth cavity wound was cut by scalpel into a porcine leg joint. The musculature in the area of the wound cavity was injected with saline to ensure that the tissue was adequately hydrated for the duration of the experiment. The wound cavity was packed with wound contacting element 14 comprising two non-woven balls of carboxymethylchitosan fibre and the wound cavity 24 and filling was covered over with the modified Allevyn Adhesive dressing described in Example 2. The vacuum source, as described in Example 2, was turned on and the vacuum tubing cup placed over the dome valve positioned centrally on the Allevyn Adhesive. Immediate contraction of the Allevyn Adhesive dressing and the wound margin was observed and the non-woven balls were compressed down to be flush with the surface of the skin. The system was left in place for 8 hours undisturbed, after which time no fluid had exited the cavity packing but had collected within it.

Example 4

Usage of apparatus based on Allevyn Adhesive as described in Example 2. A 5 cm diameter, 5 mm depth shallow wound was cut by scalpel into a porcine leg joint. The musculature in the area of the wound was injected with saline to ensure that the tissue was adequately hydrated for the duration of the experiment. The wound was covered with a non-woven sheet of carboxymethylchitosan as the wound contacting element 14 and the wound and non-woven sheet was covered over with the modified Allevyn Adhesive dressing described in Example 2 as the wound covering element 18. The vacuum source, as described in Example 2, was turned on and the vacuum tubing cup placed over the dome valve positioned centrally on the Allevyn Adhesive. Immediate contraction of the Allevyn Adhesive dressing was observed. The system was left in place for 8 hours undisturbed, after which time no fluid had exited the wound dressing but had collected within it.

Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. 

What is claimed is:
 1. A method for the application of topical negative pressure therapy to a wound site, the method comprising: positioning an absorbent material and a flexible wound covering over the wound site, the flexible wound covering being positioned over the absorbent material and the flexible wound covering being roughly level with surrounding skin; applying negative pressure to the wound site through a conduit connected to the wound covering and through an aperture in the wound covering, wherein a liquid blocking element is positioned in the aperture of the wound covering; retaining fluid from the wound site in the absorbent material; and preventing substantially all liquid from the wound site from entering the conduit with the liquid blocking element, wherein the liquid blocking element blocks substantially all liquid transport beyond itself away from the wound site.
 2. The method of claim 1, wherein preventing substantially all liquid from the wound site from entering the conduit comprises operating a valve positioned at the aperture of the wound covering.
 3. The method of claim 1, wherein the absorbent material and the wound covering are positioned over the wound site as a single element.
 4. The method of claim 1, wherein the absorbent material is configured to transform into a gel upon contact with wound exudate.
 5. A method for the application of topical negative pressure therapy to a wound site, the method comprising: positioning a wound therapy apparatus over a wound site, the wound therapy apparatus comprising: a wound covering element configured to be positioned over the wound site, wherein the wound covering element comprises an aperture, and an absorbent material positioned in or over the aperture, the absorbent material configured to transform into a gel upon contact with wound exudate; and applying negative pressure to the wound site through a conduit connected to the aperture in the wound covering.
 6. The method of claim 4, wherein the absorbent material comprises a self-coalescing material.
 7. The method of claim 4, wherein the gel is configured to absorb wound exudate.
 8. The method of claim 4, wherein the gel comprises uniform properties in each dimension.
 9. The method of claim 4, wherein the gel is a cellulosic gel.
 10. The method of claim 4, wherein the absorbent material comprises a polyanionic polymer. 